The present invention relates to an image-forming lens. In particular, it relates to an image-forming lens which serves as a photographic lens for use in TV telephones, door phones, video cameras used for monitoring or still video cameras (i.e., digital cameras).
In recent years, there has been much use of solid state photographic image elements, (e.g., CCD arrays) at the image plane of video cameras and digital cameras. These solid state photographic image elements have come to be miniaturized over the years because of progress in technology. Along with this, in the photographic image lens area, utilization has been made of wide angle lenses having a small format.
In addition, with this type of video camera or digital camera, between the photographic lens and the solid state photographic image elements, there has been increasing use of low pass filters that are used to prevent moire, infra-red light shielding filters for correcting the spectral sensitivity of the solid state photographic image elements, and glass covers for protecting the surface of the solid state photographic image elements. In such an instance it has been necessary to increase, to some extent, the back focal length Bf (hereinafter referred to simply as back focus) of the photographic lens in order to allow sufficient space for one or more of these items.
An image-forming lens having four spherical or planar surfaces is disclosed in Japanese Patent Publication No. 5-264895. The disclosed lens is known to eliminate, to some extent, the problems described above. However, according to the construction disclosed, the entire length of the system becomes extremely long, and it is difficult to achieve miniaturization. The image-forming lens disclosed in Japanese Patent Publication No. 8-5908 has two lens elements that are attached in position on the image side. With such a design, there is the problem of a dramatic increase in cost resulting from the attachment of the two lens elements, particularly with a miniature image-forming lens such as described above.